In such a system, each information is emitted in the form of light which is modulated and then introduced at the end of an optical transmission fiber and is then collected at its other end to be picked up by a receiver which reconstitutes the information. In this context, problems of aberrations are encountered mainly in introducing the light from the emitter into the fiber because, for good transmission performance, use is made most frequently of fibers whose core diameter is of the order of 5 to 50 microns; all the aberrations which lead to an image of the emitter of a greater surface area than that of the fiber core being formed on the input face of the fiber entail thereby a loss of light and a significant cause of weakening of the signal collected at the other end.
Moreover, the optical fiber telecommunication systems must usually be provided for the transmission of information in both directions. The end components therefore serve both for introducing into the fiber at least one signal from at least one emitter placed at the same end of the fiber and for extracting and separating the signals from the emitters placed at the other end. This produces some "diaphony" because stray optical signals produced in the component by the powerful light from nearby emitters can travel through the component as far as the receivers. These receivers are intended to operate at very weak optical levels because the signals which they usually receive are very attenuated by a long distance in the transmission fiber and are consequently very sensitive to any stray light. The stray light originating from the emitter and likely to reach the receivers abnormally can result, for example, from a very slight reflection on the input face of the fiber or from a return through this fiber of a reflection occurring further down the line; it can also originate from general diffusion in the optical medium as a result of imperceptible defects of homogeneity or isotropy of this medium.
For high efficiency, an end component must therefore at the same time allow the full transmission of the light from each emitter to the fiber, and avoid any stray deflection of this light toward the receivers which should collect only the light from the emitters at the other end of the fiber.